Spin transfer torque magnetic random access memory (STT-MRAM) with perpendicular anisotropy (p-STT-MRAM) has better scalability than in-plane STT-MRAM. However, the coupling field between a free layer and a reference layer in p-STT-MRAM is so strong (up to several hundred Oersteds, Oe) that the magnetization of the free layer is biased to a direction, which prefers being in parallel to that of the reference layer. Thus, the switching threshold (for current or field) is high for parallel (P) to antiparallel (AP) switching (P→AP) and low for antiparallel (AP) to parallel (P) switching (AP→P). If the coupling field is too strong, the minor hysteresis loop of the free layer completely falls in the positive (or negative) field region and loses its storage function at zero field.
On top of that, the spin current amplitude involved in PP switching is intrinsically asymmetrical due to the population difference of majority spin and minority spin. Such asymmetrical writing current brings difficulty for MRAM integrated circuit design and integration with transistor.